Author ORCID Identifier

https://orcid.org/0000-0002-0598-354X

Year of Publication

2018

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Electrical and Computer Engineering

First Advisor

Dr. Zhi David Chen

Abstract

Solar energy as the most abundant source of energy is clean, non-pollutant, and completely renewable, which provides energy security, independence, and reliability. Organic-inorganic hybrid perovskite solar cells (PSCs) revolutionized the photovoltaics field not only by showing high efficiency of above 22% in just a few years but also by providing cheap and facile fabrication methods.

In this dissertation, fabrication of PSCs in both ambient air conditions and environmentally controlled N2-filled glove-box are studied. Several characterization methods such as SEM, XRD, EDS, Profilometry, four-point probe measurement, EQE, and current-voltage measurements were employed to examine the quality of thin films and the performance of the PSCs. A few issues with the use of equipment for the fabrication of thin films are addressed, and the solutions are provided.

It is suggested to fabricate PSCs in ambient air conditions entirely, to reduce the production cost. So, in this part, the preparation of the solutions, the fabrication of thin films, and the storage of materials were performed in ambient air conditions regardless of their humidity sensitivity. Thus, for the first part, the fabrication of PSCs in ambient air conditions with relative humidity above ~36% with and without moisture sensitive material, i.e., Li-TFSI are provided. Perovskite materials including MAPbI3 and mixed cation MAyFA(1-y)PbIxBr(1-x) compositions are investigated. Many solution-process parameters such as the spin-coating speed for deposition of the hole transporting layer (HTL), preparation of the HTL solution, impact of air and light on the HTL conductivity, and the effect of repetitive measurement of PSCs are investigated. The results show that the higher spin speed of PbI2 is critical for high-quality PbI2 film formation. The author also found that exposure of samples to air and light are both crucial for fabrication of solar cells with larger current density and better fill factor. The aging characteristics of the PSCs in air and vacuum environments are also investigated. Each performance parameter of air-stored samples shows a drastic change compared with that of the vacuum-stored samples, and both moisture and oxygen in air are found to influence the PSCs performances. These results are essential towards the fabrication of low-cost, high-efficiency PSCs in ambient air conditions.

In the second part, the research is focused on the fabrication of high-efficiency PSCs using the glove-box. Both single-step and two-step spin-coating methods with perovskite precursors such as MAyFA(1-y)PbIxBr(1-x) and Cesium-doped mixed cation perovskite with a final formula of Cs0.07MA0.1581FA0.7719Pb1I2.49Br0.51 were considered. The effect of several materials and process parameters on the performance of PSCs are investigated. A new solution which consists of titanium dioxide (TiO2), hydrochloric acid (HCl), and anhydrous ethanol is introduced and optimized for fabrication of quick, pinhole-free, and efficient hole-blocking layer using the spin-coating method. Highly reproducible PSCs with an average power conversion efficiency (PCE) of 15.4% are fabricated using this solution by spin-coating method compared to the conventional solution utilizing both spin-coating with an average PCE of 10.6% and spray pyrolysis with an average PCE of 13.78%. Moreover, a thin layer of silver is introduced as an interlayer between the HTL and the back contact. Interestingly, it improved the current density and, finally the PCEs of devices by improving the adhesion of the back electrode onto the organic HTL and increasing the light reflection in the PSC. Finally, a highly reproducible fabrication procedure for cesium-doped PSCs using the anti-solvent method with an average PCE of 16.5%, and a maximum PCE of ~17.5% is provided.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2018.346

Available for download on Friday, August 09, 2019

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